1. Field of the Invention
The present invention relates to a packet switched radio telecommunication system supporting hard handover and a method for hard handover, and more particularly to a packet switched radio telecommunication system supporting mobile IP hard handover without data corruption or data loss, and a method for hard handover.
2. Background of the Invention
Mobile Internet Protocol (IP) is a standard IP that incorporates mobility of devices connected to the internet. Under Mobile IP, an IP address is allocated to a mobile terminal when a mobile terminal roams and temporarily gains access to the internet via a fixed network other than the mobile terminal's home network.
FIG. 1 is a block diagram of a network of a conventional packet switched radio telecommunication system. Mobile IP will now be described in detail with reference to FIG. 1.
Upon initial subscription, a mobile terminal (MS) 101 is connected to the Internet 106 via its home network area and receives an inherent IP address from its home agent (HA) 108. However, when the MS moves from its home network area to a foreign network area, the MS 101 is assigned a care-of address (CoA) corresponding to an IP address of the foreign agent (FA) 105 in the foreign network area and is assigned a temporary IP address from the FA 105. Then, a server 107 transmits data to the HA 108 using the inherent IP address of the MS 101, even if the MS 101 has moved outside of its home network. The HA 108 receives data intended for the MS 101, encapsulates the data, and transmits the encapsulated data to the CoA corresponding to the IP address of the FA 105 in the foreign network area. The FA 105 then transmits data to the MS 101 using to the temporary IP address. Thus, data transmitted by the server 107 to the MS 101 is routed through the HA 108 and the FA 105 in the foreign network area.
In the conventional mobile IP structure described above, when the MS 101 carries out handover, the MS 101 first transmits a handover request to the HA 108 via a support base station controller (BSC) 102, which may be combined with a packet control function (PCF), and a support packet data serving node (PDSN) 103. Further, when a target FA 105 in the foreign network area to which the MS 101 is moving is connected to a target PDSN 105 and a target BSC/PCF 104, the MS 101 disconnects a link set to the support BSC 102 and the support PDSN 103, sets a new link to the target PSDN/FA 105, and is assigned with a temporary IP address from the target FA 105. Thus, data transmitted from the server 107 is routed by the HA 108 to the MS 101 at the temporary IP address provided by the target FA 105.
The above-mentioned prior handover method has a problem in that when a MS 101 is hard handed over from a previous channel in the home network area to a new channel in the foreign network area, the connection to the home network area channel is broken before the connection to the foreign network area channel occurs. Accordingly, up-link and down-link paths are interrupted with respect to radio interface, and frame loss may result.
Korean Patent Application Publication No. 10-2001-0062319 discloses a hard handover method in which a direct link is established between the support PDSN 103 and the target PDSN 105, thereby causing a shorter disconnection with the MS 101 during the hard handover.
More specifically, a target PDSN 105 requests the support PDSN 103 for a temporary direct link. Then, when an up-link frame is transmitted from an MS 101 to a support PDSN 103 after the target PDSN 105 requests a temporary direct link with the support PDSN 103, the support PDSN 103 transmits the up-link frame from the MS 101 to the target PDSN 105 via the temporary direct link. At the same time, the support PDSN 103 transmits the up-link frame to a core network including the Internet 106. Then, the core network concurrently transmits down-link frames to both the target PDSN 105 and the support PDSN 103. The target PDSN 105 determines a handover completion time and transmits a handover command signal to the support PDSN 103 via the core network.
However, with Korean Patent Application Publication No. 10-2001-0062319, there may be delays associated with requesting and setting a temporary direct link between the support PDSN 103 and the target PDSN 105. Further, there may be delays associated with requesting the core network for handover and receiving a corresponding response. Because the request passes through the core network including the Internet 106, the delay time may be much greater. Furthermore, a network load is doubled since data is transmitted from the core network to both the target PDSN 105 and the support PDSN 103 until handover is complete.